I'm thinking the whole "Harry is going to eviscerate the entire BRF in his ghost-written memoir" is just a PR ploy to build up pre-release buzz and pre-sales. Maybe he doesn't throw his family under the bus entirely because he knows he would be cut off, his titles possibly stripped, no titles for the children, and so forth. That would basically strip his money earning potential from him for basically the rest of his life. There would be no going back. He may not be that stupid.
Of course, just the rumors about what's in the book are also incredibly damaging to his family relationships long term. They don't know what he's written and have had to wait to see. So, even if he goes easy on people individually this time, none of them know what he might do or say next year or 5 years from now, so they would be right to carefully keep their distance from him in the future. He's going to continue to be snubbed and frozen out to a great extent which, in turn, might cause him to lash out down the road.